All I Want for Christmas is… Blood.

We all know what blood is, and we’ve all at least heard about blood transfusions saving lives – but, as most of you will know, you can’t just give any old blood! Transfusion of the wrong ‘type’ of blood is incredibly dangerous, and can easily lead to death of the recipient – so typing blood correctly and understanding the differences between people’s blood is really important in a healthcare setting.

B0007649 Blood cells in a blood vessel

Once donated, blood is thoroughly tested for blood-borne infection like HIV, hepatitis and HTLV (human T-lymphotropic virus). This prevents the risk of blood transfusion recipients becoming infected from the donor blood. Blood is also typed – many of you will know your own blood type, but not so many of you might know exactly what that means.

Generally there are around 8 blood groups – O, A, B and AB, with the possibility of each being either rhesus positive or rhesus negative. These letters give basic information about the blood and allow donors and recipients to be paired together.

Being group A means that your red blood cells have ‘group A antigens’. Antigens are simply molecular markers on the surface of cells that can be recognised by the immune system. Group B means you have B antigens, group AB means you have both A and B antigens, and group O means you have neither A nor B antigens on your red blood cells.

Group A patients have ‘group B antibodies’ in their blood. Quite simply, antibodies bind specifically recognised antigens. These group B antibodies would bind to the B antigens on the blood cells of a group B patient, leading to blood coagulation in the patient which can be life-threatening. Group B patients have group A antibodies, group O patients have both A and B antibodies, and group AB have neither A nor B antibodies. (confused? See the table below!)

Blood Group A Blood Group B Blood Group AB Blood Group O
Antigens on Red Blood Cells A B A and B Neither A nor B
Antibodies in Blood B A Neither A nor B A and B

So blood with particular antigens cannot be given to those recipients whose blood contains corresponding antibody. For example, a group A donor (with A antigens) cannot give blood to a group O recipient, as the group O patient’s A antibodies would react against the A antigen on the donor cells.

The rhesus positive or rhesus negative part simply resembles another antigen. Similarly to the ABO system, being rhesus negative (Rh –) means you have no rhesus antigens but do have antibodies against rhesus antigen, and therefore a rhesus positive (Rh+) patient could receive Rh- blood without a reaction to it.

Putting this info together with the ABO system means that an ‘O Rh–’ (often just known as O+) person could give blood to any person, without fear of a reaction with the recipient’s antibodies, and so this blood type is particularly useful. On the other hand, ‘O Rh–’ (a.k.a. O-) patients can only receive blood from O+ donors, as the recipient has antibodies against A, B and rhesus antigens.

There’s a diagram below to explain who can give blood to whom! Use it alongside the table above to work it all out – it can get a little confusing! You know you really understand it when you explain why ‘O-’ donors can give to anyone, and why ‘AB+’ can only give to other ‘AB+’ people!




Storry JR, & Olsson ML (2009). The ABO blood group system revisited: a review and update. Immunohematology / American Red Cross, 25 (2), 48-59 PMID: 19927620

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